In integrated circuit technology, increasing device density has led to a decrease in the spacing between adjacent metal lines. As metal lines are formed closer together, the potential for capacitive and inductive signal interference between the lines increases. This interference is particularly troublesome when high frequency signals are propagated over adjacent lines.
In the prior art, this problem has been dealt with by constructing a metal line that is partially or totally shielded by a metal structure surrounding the metal line. A varying signal is propagated along the metal line, and the metal structure is coupled to ground potential. In combination, the two lines produce a negligible external electromagnetic field while also having a negligible susceptibility to external fields from adjacent lines. Examples of these prior art structures, as shown in FIGS. 1-5 of the present application, will now be reviewed in more detail.
U.S. Pat. No. 3,560,893 (issued 2/2/71 to Wen and assigned to RCA Corp.) discloses a partially shielded transmission line. As shown in FIG. 1 (Prior Art) the structure consists of a metal line 27 that is etched to define a central conductor 37 disposed on a dielectric substrate 25. A metal capsule 23 is bonded to the etched conductor lines 27, and is coupled to ground potential so as to partially shield inner conductor 37 from adjacent lines. The inner conductor 37 is separated from the outer conductors by air.
U.S. Pat. No. 4,575,700 (issued 3/11/86 to Dalman and assigned to Cornell Research Foundation, Inc.) discloses a partially shielded "slot-line" transmission line structure. As shown in FIG. 2 (Prior Art), conductive line 26b is deposited over a portion of a silicon substrate that is defined by two parallel grooves etched in the surface of a silicon substrate. Adjacent conductive lines 26a, 26c are formed on the opposite surfaces of the etched grooves, and the entire structure is overcoated with a dielectric 28 so as to isolate the lines from one another. Optionally, a third conductive line 40 may be formed over the structure. The conductive lines 26a, 26c and 40 are coupled to ground so as to partially shield the inset conductive line 26b that is coupled to a source of varying potential.
U.S. Pat. No. 3,370,184 (issued 2/20/68 to Zuleeg and assigned to Hughes Aircraft Company) discloses a totally shielded transmission line. As shown in FIG. 3 (Prior Art), a pair of metal (or doped silicon) lines 2, 4 sandwich a central conductor 6 having a dielectric 23 disposed about its periphery. Cadmium sulfide 8 separates the dielectric 23 from the metal lines 2, 4. The metal lines are coupled to ground and the central conductor receives a varying input signal.
U.S. Pat. No. 4,581,291 (issued 4/8/86 to Bongianni and unassigned on its face) discloses a micro-miniature coaxial conductor. As shown in FIG. 4 (Prior Art), an inner conductor 20 is formed of a copper wire having lateral dimensions in the 100 micron range. The conductor is then surrounded by a foamed dielectric 22 and a layer of chemical vapor deposited (CVD) parylene. The entire structure is then coated with an outer metal casing 24.
U.S. Pat. No. 3,351,816 (issued 11/7/67 to Sear et al and assigned to Bunker Ramo Corp.) discloses a planar coaxial type structure. As shown in FIG. 5 (Prior Art), aluminum plates 1b, 1c have a central aperture formed therein that is filled with a dielectric material 5. In turn, an aperture is formed within the dielectric material 5, and a conductive layer 7 is coated on the exposed sidewalls of the aperture. The metal layer 7 defines a through hole into which a wire 3 is inserted. The aluminum plates 1b, 1c are coupled to ground potential, and surround the central conductors 3, 7 so as to define a coaxial structure.
Reference is made to the following patents for further teachings of partially shielded transmission lines. U.S. Pat. No. 4,379,307 and U.S. Pat. No. 4,389,429 (issued 4/5/83 and 6/21/83, respectively, to Soclof and assigned to Rockwell International Corp.) disclose a conductive line that is defined and a portion of a silicon substrate that is subsequently etched so that the line is supported by a series of support ridges. As a result, the conductive line is isolated from adjacent conductors by air. U.S. Pat. No. 3,904,997 (issued 9/9/75 to Stinehelfer Sr. and assigned to Microwave Associates, Inc.) discloses a microwave transmission line formed on a dielectric body that is bonded to a conductive material having a channel formed therein. The conductive line lies within the channel, such that it is separated from the ground plane by air.
The above prior art transmission line structures pose several disadvantageous features. In the prior art structures shown in FIGS. 1 and 2, note that there is no electrical shielding provided below the central conductors 37 and 26b, respectively. While the prior art transmission line structures shown in FIGS. 3-5 do provide total shielding, the methods of manufacturing these structures are totally incompatible with current metallization processing techniques. That is, these structures would have to be formed before or after the remainder of the chip is metallized.
Accordingly, there is a need in the art to develop a totally shielded transmission line structure that is compatible with current integrated circuit metallization techniques.